Two prostatic acid phosphatase isozymes (PAP-I & PAP-II) and a glycoprotein (GP) which is immunochemically and biologically related to PAP, have been purified and partially characterized from human seminal plasma. Amino acid compositions, peptide maps and carbohydrate contents of PAP-I, PAP-II and GP have been obtained. Amino-terminal sequences of PAP-I and GP have also been determined. These chemical data as well as immunological results demonstrate that PAP-I and PAP-II are different human PAP isozymes and that the GP represents a distinct glycoprotein which shares some common enzymatic and antigenic characteristics with PAP. The antigenic structure of Human PAP has been analyzed by using three partial tryptic peptide fragments and the entire PAP molecule comprised a minimum of four distinguishable, non-overlapping antigenic determinants. Several additional human acid phosphatases have also been purified from normal lungs, spleens, kidneys and bladders, and their structural and functional properties were compared with those of human malignant PAP. The primary structure information of protein is very important in elucidating the fundamental biological function. The collaborative research of protein sequencing provides accurate information that can be used for cloning and identification of eukaryotic genes.